Many types of implantable medical devices (IMDs) have been clinically implanted into patient's bodies over the last twenty years that deliver relatively high-energy cardioversion and/or defibrillation shocks to a patient's heart when a malignant tachyarrhythmia, e.g., atrial or ventricular fibrillation, is detected. Cardioversion shocks are delivered in synchrony with a detected R-wave when fibrillation detection criteria are met, whereas defibrillation shocks are delivered when fibrillation criteria are met and an R-wave cannot be discerned from the EGM. The earliest clinically released automatic implantable defibrillators (AIDs) that were implanted in human patients provided a high energy defibrillation shock developed by an AID implantable pulse generator (IPG) through a pair of epicardial electrodes applied directly to the epicardium of the heart exposed through a thoracotomy when high heart rate detection criteria were met. Later developed and clinically implanted ICDs, originally referred to as pacemaker/cardioverter/defibrillators (PCDs), possessed more sophisticated detection algorithms and provided defibrillation, R-wave synchronized cardioversion, and pacing therapies to treat a variety of malignant tachyarrhythmias ranging from fibrillation to fast tachycardias. Current ICDs typically additionally possess single or dual chamber bradycardia pacing capabilities for treating specified chronic or episodic atrial and/or ventricular bradycardia and tachycardia. The most current clinically released ICDs also include right and left heart chamber pacing capabilities for improving the cardiac output of patient's hearts that are in heart failure. Unless otherwise indicated, all of the above-described IMDs are referred to herein as ICDs.
It was postulated early in the development of ICDs that cardioversion/defibrillation shocks could be delivered between large surface area patch electrodes implanted subcutaneously over the ribcage on either side of the heart as indicated in the article by Schuder et al. entitled “Experimental Ventricular Defibrillation with an Automatic and Completely Implanted System”, Transactions American Society for Artificial Internal Organs, 16:207, 1970. Others postulated that atrial and ventricular cardioversion/defibrillation shocks could more advantageously be applied across a fibrillating atrial or ventricular heart chamber between an epicardial electrode or the conductive case of the AID IPG and a cardioversion/defibrillation shock electrode lodged into the right atrium or ventricle, respectively, at the end of an endocardial lead advanced transvenously into the heart chamber. However, it was not possible to realize such an arrangement using lead technologies available at that time, and so the epicardial cardioversion/defibrillation electrodes were implanted initially.
The cardioversion/defibrillation leads employed with ICDs have evolved due to improvements that have been made in lead conductors capable of carrying relatively high cardioversion/defibrillation currents, electrode materials and configurations capable of distributing the energy with respect to a heart chamber, and insulating materials capable of withstanding breakdown when subjected to such energies. In particular, great strides have been made in developing small diameter, endocardial leads bearing elongated cardioversion/defibrillation electrodes that can be placed in the right heart chamber and/or into the coronary sinus that have reduced the cardioversion/defibrillation energy required to cardiovert/defibrillate the heart. Presently, clinically implanted ICDs are typically implanted with one or more endocardial lead lodging one or more cardioversion/defibrillation and pace/sense electrodes in one or more heart chamber and coronary vessel, e.g., the coronary sinus and vessels branching therefrom, in conjunction with use of the ICD IPG housing or “can” electrode or a further subcutaneous cardioversion/defibrillation electrode as a further remote electrode.
The implantation of such endocardial cardioversion/defibrillation electrodes used in conjunction with subcutaneous cardioversion/defibrillation electrodes does eliminate the trauma associated with surgically accessing the epicardial surface to implant epicardial cardioversion/defibrillation electrodes. Nevertheless, it has long been appreciated that the subcutaneous implantation of the ICD and all of the associated cardioversion/defibrillation leads and electrodes would also advantageously simplify the procedure and reduce the expense of implantation.
Consequently, the possibility of implanting at least two large surface area cardioversion/defibrillation electrodes subcutaneously and coupling the electrodes to a subcutaneously implanted ICD IPG has been revisited on a number of occasions since the first such implantation by Schuder. For example, the disclosures of U.S. Pat. Nos. 5,255,692 and 5,342,407 and in U.S. Patent Application Publication Nos. 2002/0042634 and 2002/0035377 to Bardy et al., evidence such continued interest.
In the '407 patent, the disclosed defibrillation system includes an IPG and a pair of leads connecting the IPG to defibrillation electrodes that are implanted subcutaneously outside of the rib cage in the thoracic region on opposite sides of the heart. More particularly, one defibrillation electrode is subcutaneously implanted to the left of, and anterior with respect to, the heart, and the other defibrillation electrode is subcutaneously implanted posterior with respect to the heart, and to the right of the heart. The IPG is also implanted anterior and to the left of heart, below one of the subcutaneous defibrillation electrodes. The IPG can incorporate circuitry for sensing cardiac electrical activity, in which case the same large surface area, subcutaneous defibrillation electrodes are used for sensing such activity as well as delivering defibrillation pulses.
A similar defibrillation system is disclosed in the '692 patent, except that the defibrillation electrodes are shaped and sized to conform to an interspace between the periost and the bone at the inner side of a rib proximal to the heart, e.g. the fourth rib. The shaped defibrillation electrodes also have a fixation mechanism that engages the ribs and is used to maintain the electrode in position chronically.
The above-referenced Bardy et al. applications disclose subcutaneously implanted ICD IPGs that are coupled with at least one cardioversion/defibrillation lead. In certain embodiments, the ICD IPG has a conventional configuration having a can electrode that functions as one cardioversion/defibrillation electrode and is implanted subcutaneously anterior or posterior to the heart. The cardioversion/defibrillation lead is tunnelled subcutaneously under the skin and around the thorax to locate the lead supported cardioversion/defibrillation electrode posterior or anterior to the heart, respectively. In certain embodiments, two cardioversion/defibrillation leads that are electrically connected together are tunnelled subcutaneously under the skin and around the thorax to locate the two cardioversion/defibrillation electrodes apart from one another and posterior or anterior to the heart, respectively. Electrical sensing of the cardiac EGM is accomplished across two sense electrodes displaced apart from one another on the IPG housing. Cardioversion/defibrillation shocks are delivered across the thorax between the cardioversion/defibrillation electrodes on the IPG housing and the lead. It is also asserted that cardiac pacing pulses can be applied to the heart across the cardioversion/defibrillation electrodes on the IPG housing and the lead. In certain embodiments, the IPG housing is shaped in an elongated, thin, narrow shape to approximate and conform to the curvature of the thorax for cosmetic reasons and in some cases to fit between the ribs, e.g., between the fourth and fifth ribs.
In commonly assigned U.S. Pat. No. 5,314,451, an ICD IPG is disclosed that is adapted to be coupled to a three-conductor cardioversion/defibrillation lead of any of the known types. The ICD IPG is formed in two separate hermetically sealed housings or cans that are coupled together by a power delivery cable. A first hermetically sealed can encloses the electronic circuitry of the ICD as well as the small volume, long-lived pacing and sensing battery, and the second hermetically sealed can encloses the high voltage, large volume cardioversion/defibrillation shock battery. The second can enclosing the large volume cardioversion/defibrillation shock battery can be readily surgically accessed and replaced upon depletion of the large volume cardioversion/defibrillation shock battery while leaving the cardioversion/defibrillation lead and the first can enclosing the IPG circuitry undisturbed. The cardioversion/defibrillation leads coupled with the first can extend transvenously into a heart chamber or cardiac vessel. Pacing and cardioversion/defibrillation therapies are generated by the electronic circuitry of the first can and delivered to the heart through pace/sense and cardioversion/defibrillation electrodes on the lead and a common electrode on the surface of the first can in any of the conventional manners.
In one disclosed embodiment, a first cardioversion/defibrillation electrode is formed on the first can and a second cardioversion/defibrillation electrode is formed on the second can. The first and second cans are adapted to be implanted subcutaneously in the thoracic region, whereby the first and second cardioversion/defibrillation electrodes are disposed apart from one another and with respect to the heart. How cardioversion/defibrillation shocks are delivered employing the first and second cardioversion/defibrillation electrodes is not explained, but the first and second cardioversion/defibrillation electrodes could be electrically coupled to the COMMON output of the high voltage output circuitry to be used in conjunction with biphasic shocks delivered from the HV-1 and HV-2 terminals between the commonly connected subcutaneous cardioversion/defibrillation electrodes and the cardioversion/defibrillation electrode on the lead inserted into the heart chamber.
There are a number of appreciable drawbacks to simply delivering cardioversion/defibrillation shocks between subcutaneously implanted cardioversion/defibrillation electrodes of the types described in these patents and applications. First of all, the energy demands on the high voltage cardioversion/defibrillation battery and the low voltage pace/sense battery are magnified considerably by the additional impedance presented by the body tissue and fluids between the spaced apart subcutaneous electrodes. Pacing pulse energies may have to be increased ten-fold to capture the heart. Pacing at these high energies is known to be painful. And, only simple ventricular pacing algorithms may be usable to overdrive the heart. Similarly, the magnitude of cardioversion/defibrillation shocks may be such as to be extremely painful to the patient, if the patient is still conscious when the shock is delivered. On the other hand, there are several reasons why a simple, inexpensive, subcutaneously implanted ICD would be of benefit.
First of all, highly sophisticated testing and monitoring equipment and trained electrophysiologists are presently required to implant the current highly sophisticated and expensive ICDs. Electrophysiologists or specialized cardiologists are required to work up the patient, particularly to induce a malignant tachyarrhythmia, e.g., fibrillation, so as to ascertain the parameter of the arrhythmia detection algorithms that will reliably result in a declared tachyarrhythmia, to ascertain which therapies are to be delivered that will reliably cardiovert or defibrillate the heart in response to the declared tachyarrhythmia, the shock energy of each delivered cardioversion/defibrillation shock or other therapy that is to be delivered, and to otherwise perform the surgical procedure. There are not enough trained electrophysiologists or cardiologists to perform these procedures, particularly in less developed countries. And, the ICD cost and procedure cost must be reduced to make the therapy available in such countries.
Therefore, a need exists for simplified and less expensive ICDs that can be implanted by cardiologists and general surgeons to meet the needs of such population groups.
Secondly, because such ICDs and procedures are highly expensive, most patients in developed countries who receive the same have experienced and survived a sudden death episode. Survivors of sudden death episodes are in the minority, and so studies are ongoing to identify patients who are asymptomatic by conventional measures but are nevertheless at risk of sudden death. Current studies of patient populations, e.g., the MADIT II and SCDHeFT studies are establishing that there large numbers of patients in any given population that are susceptible to sudden death and that they can be identified with some degree of certainty. However, implanting currently available ICDs in all such patients would be prohibitively expensive.
Therefore, a need exists for an inexpensive, simplified, prophylactic ICD that can be subcutaneously implanted in such patients in the expectation that the patient may at some time suffer a sudden death episode, causing the ICD to deliver the appropriate cardioversion/defibrillation shock and possibly to deliver post-shock pacing, if necessary. Such a subcutaneously implanted ICD would likely be capable of delivering a limited number of cardioversion/defibrillation shocks at maximal shock energy and deliver pacing pulses for a few minutes time. In this way, the patient can be assured of surviving the first sudden death episode. The prophylactic ICD in a surviving patient would then be replaced by the more sophisticated, longer-lived current ICD.
There are also pediatric patients whose hearts are too small or are naturally growing and cannot accommodate transvenous cardioversion/defibrillation leads. In certain cases, cardioversion/defibrillation leads have been implanted subcutaneously rather than in the heart chambers in such patients. See Gradaus et al., “Nonthoracotomy implantable cardioverter defibrillator placement in children: use of subcutaneous array leads and abdominally placed implantable cardioverter defibrillators in children”, J Cardiovasc Electrophysiol, 12: 356-60, 2001.
Current ICD IPGs are relatively heavy and bulky compared with pacemaker IPGs despite substantial reductions in weight and volume that have been achieved over the years since the first AID IPGs were implanted. The ICD IPG comprises the hermetically sealed housing and a connector header or block attached to the housing for making connection with the cardioversion/defibrillation and pacing leads. The bulk of a given ICD IPG connector block depends upon the number of lead connectors and connector elements that are to be attached to it. The components within a given hermetically sealed housing that dictate its bulk and weight comprise the ICD circuitry, a relatively low voltage battery providing operating energy to the ICD circuitry and providing pacing pulse energy, a high current battery providing cardioversion/defibrillation shock energy, a charging transformer, a set of discrete diodes and high voltage capacitors that are charged through the transformer and diodes and discharged through the cardioversion/defibrillation electrodes, a telemetry antenna, feedthroughs to connector elements in the connector block, component spacers for holding the components in a sub-assembly, and an activity sensor in certain cases. The largest volume and weight of components in the typical ICD IPG include the set of high voltage capacitors, the high current battery, and the connector block. Current ICD IPGS range in weight between about 75 grams and about 115 grams and range in volume between about 36 cc and 62 cc. The weight and volume is proportional to the specified available cardioversion shock energy, which can range between about 25 joules and about 40 joules. The volume and weight of a given ICD IPG is therefore dependent upon the capacity of the high voltage battery, the specified energy output, and the projected battery life, assuming a standard number of shock deliveries and capacitor reformations per year.
Such relatively bulky and heavy ICD IPGs are normally implanted subcutaneously over the abdomen or the pectoral regions rather than between the skin and ribs of the thorax since the former regions have thicker subcutaneous fat, tissue and muscle layers. The outline of the implanted IPG housing is less apparent and cosmetically offensive to the patient. A great deal of design effort has been expended in achieving the most efficient and lowest volume shape of the ICD IPG housing, connector block, and the above-listed components within the hermetically sealed housing having as thin a profile as possible to make it as invisible and comfortable as possible after implantation.
As noted above, it has been found desirable to implant the ICD IPG between the skin and ribs in posterior or anterior implantation sites to function as one cardioversion/defibrillation electrode and to route the cardioversion/defibrillation leads connected to the IPG connector block subcutaneously around the thorax to locate the cardioversion/defibrillation electrodes at a remote subcutaneous implantation site. As further noted above, it is proposed to specifically narrow and elongate the cardioversion/defibrillation electrodes and ICD IPG housing to conform more closely to the space between the ribs than is possible with the typical cardioversion/defibrillation electrodes and ICD IPG housing. However, volumetric efficiency is sacrificed when such shape changes are made, and volume and weight of the ICD IPG may actually increase.
Therefore, for these and other reasons, a need exists for a relatively simple, cosmetically un-intrusive ICD that can be implanted beneath the skin over the ribcage with minimal trauma to the patient by surgeons employing conventional surgical instruments and monitoring equipment so as to make the implantation less expensive and more widely usable.